1. Field of the Invention
The present invention relates to communications. More particularly, the invention concerns a method and apparatus for transmitting information in a wireless communication system.
2. Description of the Related Art
FIG. 1 illustrates a portion of the radio frequency spectrum used in a common telecommunications system. Frequency range 100 centered around 800 MHz has historically been known as the cellular frequency range and frequency range 102 centered about 1900 MHz is a newer defined frequency range associated with personal communication services (PCS). Each range of frequencies, i.e., the cellular and PCS, are broken into two portions. In the cellular frequency range 100, there is a reverse link portion 104 that is used for communications from a mobile communication device to a base station such as a cellular base station. Portion 106 of cellular frequency range 100 is used for forward link communications, that is, communications from a cellular base station to a mobile communication device. In a similar fashion, portion 108 of PCS frequency range 102 is used for reverse link communications, that is, communications from a mobile communication device to a base station. Portion 110 of PCS frequency range 102 is used for reverse link communications, i.e., communications from a base station to a mobile communication device.
Each of the frequency ranges is broken into bands that are typically associated with different service providers. In the case of cellular frequency range 100, frequency bands 112 and 114 are designated band “A” for reverse link and forward link communications, respectively. A reverse link is the band connecting a mobile station to a base station, and a forward link is the band connecting a base station with a mobile station. In a particular geographic area, a cellular service provider is assigned frequency band “A” in order to carry out mobile communications. Likewise, in the same geographic area another cellular service provider is assigned frequency bands 116 (for forward link communications) and 118 (for reverse link communications) which are designated band “B”. The transmit and receive frequencies are separated by 45 MHz and with the minimum separation between transmit and receive bands is 20 MHz. This minimum separation is to avoid interference between the forward and reverse links and to permit diplexers, which separate the forward and reverse link signals in a mobile station to be used.
A few years ago, the US Government auctioned the PCS frequency spectrum to service providers. As with the cellular frequency range, the PCS frequency range is broken into several bands where a different service provider may use a particular frequency band for which it is licensed within a particular geographical area. The PCS bands are referred to as A, B, C, D, E and F. The A band includes reverse link band 120 and forward link band 122. The B band includes reverse link band 124 and forward link band 126. Band C includes reverse link band 128 and forward link band 130. The reverse link and the forward link band of A, B and C bands are each 15 MHz wide. The D band includes reverse link band 132 and forward link band 134. The E band includes reverse link band 136 and forward link band 138. Likewise, band F includes reverse link band 140 and forward link band 142. The reverse link and forward link bands of D, E and F are each 5 MHz wide. Each of the different cellular and PCS bands can support a number of communications carriers in both the reverse link and forward link direction.
As shown in FIG. 1, it is possible to have as many as eight different wireless communication service providers in a particular area—two cellular service providers, each having a total allocated bandwidth of 25 MHz (forward and reverse links), and six PCS service providers, each having a total allotted bandwidth of 30 MHz for the A, B, and C blocks or 10 MHz for the D, E, and F blocks. These providers may employ different technologies for transmitting and receiving telephone calls, data, control commands, or other types of information, singularly and collectively referred to in this application as information signals. For example, a time-division-duplexing technique, a frequency-division-duplexing technique, or a code-division-multiple-access (CDMA) technique might be employed by a provider as described below. Further, if the carrier is using CDMA, then various CDMA releases are available such as IS-95-A and IS-95-B.
Recently, in response to consumers demand for greater service options, the International Telecommunications Union (ITU) solicited proposals for Third Generation wireless communications. The Third Generation Proposals strive to expand the capabilities of the preceding technologies to include wireless e-mail, Web browsing, and corporate and local network access, as well as videoconferencing, e-commerce and multimedia. One of the candidate submissions to the ITU was proposed by subcommittee TR45.5 of the Telecommunications Industry Association (TIA) and was called cdma2000, which has since been developed and continues to be developed under the name of IS-2000. The proposed cdma2000 system includes three modes of operation: 1×, 3× direct spread (DS) and 3× multi-carrier (MC). Each of these modes can be operated in frequency division duplex (FDD) or time-division duplex (TDD) manner.
The 1×FDDmode operates within a 1.25 MHz bandwidth on both the forward and reverse links, thereby providing for higher capacity in the 1.25 MHz bandwidth and supporting high-speed data transmissions. The spreading rate is 1.2288 Mcps on both the forward and reverse links of 1× systems. The 3×FDDmode operates within a 3.75 MHZ band on both forward and reverse links. The 3× mode forward link employs either a direct spread or a multi-carrier transmission format. In the 3× direct spread mode, a single forward link carrier with a chip rate of 3.6864 Mcps is used; in the 3× multi-carrier mode, the forward link consists of three carriers that are each spread at a spreading rate of 1.2288 Mcps. The 1×TDD mode operates within a single 1.25 MHz bandwidth for both the forward and reverse links. The 3× direct spread and multi-carrier TDD modes operates within a single 3.75 MHz for both the forward and reverse links.
By using the 3×FDD mode and providing a forward link using the multi-carrier format, a communications system is fully compatible with existing IS-95 system. That is, the cdma2000 forward link structure may be “over-laid” on existing PCS systems. One attribute that makes the forward link multi-carrier system compatible with existing systems is that it preserves orthogonality of signals transmitted in the forward link. The reverse link is not orthogonal, so cdma2000 systems use a direct spreading to 3.6864 Mcps. When used, the time-division-duplex (TDD) mode of operation allows both the forward link and reverse link to be transmitted in a single 1.25 MHz band. The TDD forward link is transmitted in a first time interval and the TDD reverse link is transmitted in a non-overlapping second time interval. The transmissions in both time intervals are direct spread at a 1.2288 Mcps spreading rate.
As mentioned above, Third Generation Systems such as cdma2000 3× are designed for transmitting information that may have very high data transfer requirements, such as email downloading and web browsing. For example, a mobile station user may send a simple message requesting that a page from web site be downloaded to his mobile phone. This simple request requires very little bandwidth when transmitted on the reverse link to the base station, but timely downloading of the web site on the forward link from the base station to the mobile station will require substantial bandwidth. A request for a page may be in the order of a few hundred bytes, but the response from the web server can be several tens of thousands of bytes, particularly if it includes graphics or pictures. However, in the currently proposed Third Generation Systems, the bandwidth allocated to reverse link transmissions is the same as the bandwidth allocated for forward link transmissions.
What is needed is a method and apparatus that will allow for the bandwidth allocated to the forward link to be different than the bandwidth allocated for the reverse link. One version of the method and apparatus should provide for better spectrum management. Further, the method and apparatus should allow the user of a technology such as cdma2000 1× to easily transition to a newer version of the technology, such as cdma2000 3×.